Maintaining Grain in Storage
October 01, 1996
by Teresa Acklin
Regulating temperature and moisture prevents damage to valuable grain inventories.
By Dale Fehrenbach
Grain handling and storage practices affect everyone in the industry, from grain buyers and elevator managers to equipment suppliers and processors.
The grain industry handles huge quantities of inventory every year. Unfortunately, because of the relatively low value per unit of inventories, we sometimes get complacent, don't properly educate ourselves or don't consider all of the risks and variables in storing and handling grains and oilseeds.
This often leads to disastrous results in the form of quality degradation, weight loss and the resulting financial negatives. In the worst cases, personnel and facility safety can be compromised.
Numerous real-life examples exist of the disastrous results of poor grain quality management. Several years ago, a large covered pile of maize began heating in the center spoutline. Efforts were made to remove the center heating spot, but when the center was exposed to air, it was so hot that combustion occurred. In the reclaim effort, smoke odor, damaged grain, water and snow contaminated virtually all of this pile. More than a million U.S. dollars of loss occurred.
Here's an extreme example of storing soybeans too wet, not properly caring for them and not monitoring their condition. After an extended period of storage, the bottom slide gates on a bin were opened. Nothing came out of the bin. Subsequent examination showed that of the more than 5,000 tonnes of soybeans, only the top surface and a few small pockets were not severely heat-damaged. This condition problem cost nearly a million U.S. dollars and, as with the maize pile, brought some serious safety risks.
Insect damage usually isn't as readily visible or visually dramatic, but the losses in quality and quantity are just as large.
Resolving some critical issues will help grain handlers avoid the catastrophes as well as the smaller losses that chip away at profits and cause us to not meet customer requirements. One of two problems is associated with virtually all stored grain deterioration, and these are molds and insects.
Mold and Insect Damage
When mold spores exist which is a given in grain handling and moisture and temperature conditions are conducive, mold growth will occur. That mold growth results in a chemical reaction that gives off heat, water and carbon dioxide as a by-product of its respiration. The heat and water produced in the chemical reaction often perpetuate the process.
There are at least six types of storage molds that can occur. Each of these has slightly different characteristics and results. The most important thing to know about these molds is that their growth is very moisture-dependent. They grow very rapidly at moisture levels above 16%, and they grow very slowly at moisture below 14.5% in maize.
Grain temperature also has a major controlling factor over mold growth, which readily occurs between 10°C and 40°C; below 7°C it is practically dormant.
Insects in stored grain come in two broad varieties, those that are internal feeders and those that feed externally. The internal feeders consist primarily of several varieties of weevils and grain borers. This type of insect bores into kernels of grain, consuming parts of the kernel. The respiration of these insects produces moisture and heat, which readily cause mold growth and resulting damage.
The external feeders are primarily bran bugs and Indian meal moths. These insects survive by eating the dust and other very small grain fragments and mold. External feeders' respiration produces some moisture and heat, and mold begins to grow. Also, heavy infestations cause a musty odor.
Like molds, insects have a preferred environment in which they are much more active. The types of insects that are injurious to stored grain are very active at about 16°C to 38°C. From 16°C down to about 4°C, their activity is relatively slow, and below 4°C they are essentially dormant.
Temperatures in stored grain at the high end of this scale are not favored by insects, but excessive mold growth certainly can heat grain well beyond these temperatures. If insects are present when grain temperatures begin exceeding 38°C, they will move to a cooler area if it's available, or they will die.
That level of temperature for high insect activity is nearly identical to the preferred environment of molds. Moisture levels above about 11.5% provide the environment that is conducive to insect activity. Recall that mold activity really shuts down at about 14.5%, so having grain at a moisture level that hinders mold growth doesn't necessarily hold down insect activity.
Grain temperature and moisture are the main culprits in controlling grain quality degradation. The good news, though, is that there are tactics to control both mold and insect activity.
Using Air Flow
Aeration is the use of forced air flow through a grain mass to remove small amounts of moisture, cool the grain via evaporative and conductive cooling and improve the uniformity of both grain moisture and temperature. There are two principle types of aeration.
Upflow aeration provides less pressure drop through the ducts and is useful to begin cooling a bin prior to its complete fill. The disadvantage of upflow aeration is that air passing through a fan, then into the bin, is slightly warmed, so you don't get the full benefit of cool ambient air.
Upward air flow also requires good forced air mixing on the top surface of the grain. Where aeration air travels up through the grain mass and reaches the surface of the grain, it can be moisture-laden and often will be warmer than ambient air in the over space. This featrure causes moisture condensation on the surface of the grain and/or under the surface, a condition which creates a great environment for mold growth.
For example, maize at 21°C and 15.5% moisture is in equilibrium with 21°C air at 78% relative humidity. The dew point for this air is 16°C, so that if the air comes in contact with the underside of the roof at 16°C or less, moisture will condense out and drip back into the grain. Mixing or exhaust fans on the roof of a bin can pull this surface moisture off via dilution with low humidity air or evaporation.
Downward aeration is advantageous in that mixing or vent fans on the top of the bin are not needed, and inlet air is not warmed by the fans. The disadvantage of downward air flow is that you can't obtain its full effectiveness until a bin is full. There generally is more pressure drop through the ducts on down draft systems.
Spoutlines are a major nemesis in the storing of grain. These areas, under the fill points of bins, have high concentrations of fines and foreign material, which often are wetter than the grain. That, in combination with the inability to cool this with aeration because the fines block the air flow, almost guarantees a condition problem. The only viable solution is removing the spoutline, cleaning it and replacing it or leaving that area empty. A temperature cable in the spoutline, or possibly several cables, should be considered mandatory.
Regulating Seasonal Affects
There are some differing opinions about whether to run aeration at different times of the year and in different circumstances. I believe grain that is to be stored over winter should be cooled to approximately 4°C if possible. Cooling to lower temperatures is generally a waste of energy for no benefit. It is advisable to occasionally aerate during winter months to reuniform the temperature of the grain mass.
Taking grain from cold weather months into the warmer months offers some risks that need to be carefully thought through. Generally, I would aerate grain to about 16°C and then stop aerating. This 16°C temperature might be slightly higher in warmer climates. A practice like this is a compromise between keeping the grain at a temperature that's in the lower range of mold and insect activity and one that will be relatively close to ambient temperature ranges. Keeping the grain at 4°C cold-weather temperature will result in excessive sweating on the surface and the walls of bins. Warming the grain to 21°C or higher encourages mold and insect activity much more so than leaving it at 16°C.
An example of poor judgment in the use of aeration involves two distinct temperature levels in a bin. This can occur when pull-down aeration is used and the aeration process is stopped prior to cooling the entire grain mass. Where these temperature extremes exist, condensation, mold growth and quality degradation will occur.
A second frequently overlooked issue is capping of aeration fans. Cool ambient air can access the grain-warmed duct in the bottom and cause condensation and crusting of the grain near the ducting. This will partially or completely stop air flow through the ducting.
In addition to the improper use of aeration, we can create major temperature differences by the way we bin grain. This will also result in condensation and mold activity where these temperature extremes clash.
Properly cooled grain is not a guarantee of trouble-free storage. Take the case of warm grain coming into contact with much colder conditions near the outside walls of a bin, and the grain and air just inside the bin wall cools.
This air becomes heavier, falls and sets up an air migratory pattern that causes the warmer air to rise in the center of the bin. When that air hits the surface of the grain, which is cooler, moisture is dropped and ideal mold growth conditions exist.
When taking storage grain from cold to warm months, a similar situation occurs. Air being warmed near the outside walls rises and sets up a migratory pattern.
Considering the Variables
Prior to putting grain in storage, numerous variables should be studied. Some of these considerations, such as grain moisture, the uniformity of moisture, the quality blend and the amount of mold in the grain, may be totally out of the control of operators.
We can use those factors as excuses or be proactive anticipating grain quality losses by educating ourselves, by constantly monitoring stocks, by reacting to signals of impending problems and then by working with merchants and customers to make proper risk/reward decisions.
From a practical standpoint, grain quality management is part science and part art or experience. The low unit price of our inventories may allow complacency, and I believe that many of us allow small losses in the form of quality degradation, or weight losses due to quality degradation or over-aeration. We also elevate per unit variable expenses in wasted energy. These small, per unit losses or expenses in total can be huge, often not very visible but always preventable.
From my experience, this is an area where we as operators of facilities can make a tremendous contribution to the success of our companies.
Dale Fehrenbach, a past president of the Grain Elevator and Processing Society, is assistant vice president, plant operations manager of Cargill's Grain Division in Minneapolis, Minnesota, U.S. This article is based on a presentation to the GEAPS Exchange '95 conference.